The present invention relates to a scanning electron microscope.
A known electron detector used in a scanning electron microscope includes a scintillator, a photomultiplier tube, and a light guide that connects the scintillator and the photomultiplier tube to each other. The electron detector detects an electron (a secondary electron or a backscattered electron) generated by irradiating a sample with an electron beam.
For example, U.S. Pat. No. 6,545,277 discloses an electron detector configured such that a through-hole is provided in a scintillator to enable a primary beam (an electron beam for irradiating a sample) to pass through the through-hole. A liner tube is inserted into the through-hole provided in the scintillator, and the primary beam passes through the liner tube.
With an electron detector, sufficient light emission for detection occurs in the scintillator as long as an electron has high energy when incident on the scintillator. However, since sufficient light emission does not occur when the energy of an electron is low when incident on the scintillator, detection becomes difficult. For example, Japanese Patent Application Publication No. 2013-58314 describes that, when energy of an electron upon incidence to a scintillator is lower than 5 keV, sufficient light emission does not occur and detection becomes difficult.
In order to address such a problem, in Japanese Patent Application Publication No. 2013-58314, a surface of the scintillator is uniformly coated by a conductive layer (a conductive body) and a positive voltage of 5 keV or higher is applied to the conductive layer. Accordingly, even when the energy of an electron is lower than 5 keV, since the electron is accelerated before reaching a sensitive surface of the scintillator and sufficient light emission occurs, detection can be performed.
FIG. 12 is a graph illustrating a potential distribution on a surface of a scintillator in a state where a conductive layer is not formed on the surface of the scintillator. Moreover, in FIG. 12, an abscissa represents a distance in a radial direction from an optical axis and an ordinate represents a voltage on the surface of the scintillator. In FIG. 12, a voltage is applied to a circumferential portion of the scintillator.
As is apparent from the result illustrated in FIG. 12, with a detector in which a liner tube is inserted into a through-hole provided in a scintillator as described in U.S. Pat. No. 6,545,277 mentioned above, when a conductive layer is not formed on a surface of the scintillator, since voltage is low in a vicinity (around r<4) of the liner tube, detection efficiency of electrons declines significantly.
However, with a detector in which a liner tube is inserted into a through-hole provided in a scintillator, when a conductive layer is formed on a surface of the scintillator, since a potential difference is created between the conductive layer and the liner tube, a discharge between the conductive layer and the liner tube becomes an issue.
Since the conductive layer formed on the surface of the scintillator must allow electrons to pass through (must transmit electrons), the conductive layer is formed extremely thinly. As a result, a resistance of the conductive layer is large. Therefore, when a discharge occurs between the scintillator and the liner tube, there is a risk that a delamination of the conductive layer on the surface of the scintillator may occur due to resistive heating during the discharge.
Although the possibility of an occurrence of a discharge can be sufficiently reduced by increasing the distance between the scintillator and the liner tube, this means that an area of a sensitive surface of the scintillator is to be reduced.